In simulcast systems it is important that identical modulating signals are produced at each transmitter site. Differential delays as small as tens of micro-seconds between the signals can significantly reduce coverage. As is known, a digital signal processor (hereinafter "DSP") may be used to produce the modulating signals at the sites. Digital technology creates identical digital signals at each site. However, a new problem is created by this method. In order to interact with the analog world, the digital filter must pass through a digital to analog (hereinafter "D/A") converter and anti-alias filter. The latter device is an analog device which is subject to the tolerances of the components used in manufacturing. Using 5% tolerance components can produce as much as 20 micro-seconds of delay in a 5-pole 10-kHz filter.
There are several obvious solutions with significant costs attached. One solution is to use more expensive parts with higher tolerances to reach the desired specifications. Another solution is to use a wider anti-alias filter and increase the sampling rate. The higher sampling rate would increase the loading on the DSP and reduce available processing capacity for other features.
In the manufacture of analog devices such as filters, there is always a margin of error in the performance as a result of tolerances in the manufacturing process. In a simulcast system, this margin of error can significantly reduce the coverage area. The invention disclosed herein allows exact digital technology to compensate for the error introduced by an analog device.
The concept of digitally reconstructing the modulation signals at the remote sites is a new concept and therefore the problem of differential delay has not been previously encountered. In past systems the modulating signal would be created at the central site and transmitted over wide-band communication lines. However, adaptive filter are often used in modems where they equalize the distortion induced in the communication link. In the modem application, a predetermined training sequence is sent prior to a data transmission by the source. The receiver calculates the distortion induced by the communication link and adapts accordingly.
In all digital systems, a D/A converter must be followed by a recovery filter and an analog to digital (hereinafter "A/D") converter must be preceded by an anti-alias filter to guarantee basic laws of the signal processing are satisfied. The key circuit improvement of this invention is the use of the anti-alias filter for both analog recovery and analog sampling. This dual role is allowed since the sample clock and recovery clock are running at the same frequency. The use of DSP's such as the Motorola model 56000 aid this invention by making the implementation of the adaptive algorithm relatively effortless.